The transmission of soliton pulses or "solitons" in the portion of an optical fiber that has abnormal dispersion is a known phenomenon. The transmission of so-called "black" solitons constituted by pulse "holes" in a continuous signal, in the normal dispersion portion of an optical fiber is also known; in this case, the solitons have a wavelength such as to propagate with negative chromatic dispersion. Both for "white" solitons and for "black" solitons, to compensate dispersion of the optical signal, use is made of the non-linearity in the corresponding portion of the fiber. Soliton transmission is modelled in known manner by the non-linear Schrodinger equation.
Various effects limit the transmission of such pulses, such as jitter induced by solitons interacting with the noise present in the transmission system, e.g. as described in the article by J. P. Gordon and H. A. Haus, published in Optical Letters, Vol. 11, No. 10, pp. 665-667. This effect, known as the Gordon-Haus effect, puts a theoretical limit on the quality or the bit rate of transmission by means of solitons.
Because of the deformations induced on solitons during transmission, and in particular because of the jitter induced by the Gordon-Haus effect, considerable efforts are needed to ensure that a signal encoded by solitons is transmitted, and to make it possible to recover the necessary clock frequency from pseudo-random signals. Thus, sliding guiding filter systems have been proposed that enable the jitter of transmitted solitons to be controlled, as have various clock recovery systems, both optical and optoelectronic. Such systems are relatively expensive and complex, in particular because of the need to eliminate the effects of jitter before recovering the clock. An example of such a clock recovery system is described in FR-A-2 706 710.
As described by F. M. Mitschke and L. F. Mollenauer, Optical Letters, Vol. 12, No. 5, pp. 355-357, adjacent solitons interact. This interaction appears as attraction between adjacent solitons in the absence of modulation, i.e. for solitons that are in phase. It appears as repulsion between adjacent solitons when they are in phase opposition.
This interaction is generally considered as being a harmful phenomenon since it leads to deformation of transmitted solitons that can lead to loss of information, see for example N. J. Smith et al., Optical Letters, Vol. 19, No. 1, pp. 16-18, which presents such interaction as "one of the major constraints in the design of soliton optical fiber communications systems". In the prior art, proposals have been made to avoid this interaction by imposing a constraint on the time "distance" between two transmitted solitons, thereby limiting the effects of interaction between solitons. A commonly accepted value for the minimum separation between two solitons is 0.2.times.Dt where Dt is the width of the solitons, conventionally defined by energy being equal to half the maximum energy, and known as "full width at half maximum" (FWHM).